Polymeric aromatic carbamic acid esters (polyurethanes) such as diphenylmethane dicarbamates and the related higher homologs polymethylene polyphenyl carbamates have become increasingly important products, particularly for use in the preparation of the commercially valuable diphenylmethane diisocyanates and mixtures of diisocyanates and polyisocyanates by the decomposition of such polymeric aromatic carbamic acid esters in a suitable solvent as shown, for example, in Rosenthal et al, U.S. Pat. Nos. 3,962,302 and 3,919,279.
At the present time there is no known successful commercial method for the direct preparation of polymeric aromatic esters of carbamic acid. The corresponding diphenylmethane diisocyanates and polyisocyanates, available commercially, are largely produced by the phosgenation of mixtures of diamines and polyamines obtained by the condensation of aniline and formaldehyde with catalytic quantities of a mineral acid, as for example, disclosed in the Pistor et al, U.S. Pat. No. 4,014,914.
Prior art processes have been proposed for the preparation of polymeric aromatic carbamic acid esters (polyurethanes) as for example in Klauke et al, U.S. Pat. No. 2,946,768 and British Pat. No. 461,352 which disclose the condensation of aryl carbamic acid esters with carbonyl compounds such as aldehydes and ketones in a dilute aqueous mineral acid condensation medium. In such processes the carbonyl compound such as formaldehyde tends to react at the nitrogen of the carbamate to produce along with some desired polyurethanes, varying amounts, i.e., generally between 15 percent and 50 percent by weight, of undesirable N-(alkoxycarbonyl)phenylaminomethylphenyl compounds which include dimers, trimers, tetramers, etc. of such compounds, which compounds referred to as N-benzyl compounds are fully described in co-pending allowed U.S. application, Ser. No. 905,705, filed May 15, 1978. Attempts to prepare diisocyanates and polyisocyanates or to otherwise use the mixture containing the polyurethanes and such amounts of the undesired compounds, which compounds cannot be converted to an isocyanate by pyrolysis, presents many problems since there is no known method for separating the polyurethanes from the N-(alkoxycarbonyl)phenylaminomethylphenyl impurities.
The present invention which comprises the preparation of diphenylmethane dicarbamates and polymethylene polyphenyl carbamate homologs and derivatives of these compounds by the condensation of N-aryl carbamic acid esters with formaldehyde, para-formaldehyde or trioxane in the presence of an organic sulfonic acid which has an acid concentration of at least 75 percent substantially avoids the above mentioned problem and the formation of such impurities.
The organic sulfonic acids employed in the process of this invention are superior catalysts for the condensation of the N-aryl carbamates for a number of reasons. (1) Containing organic substituents the sulfonic acids are generally more soluble in the carbamate solution than the mineral acids thus providing a single phase reaction at the operable concentrations. (2) The organic sulfonic acids which are strong acids substantially eliminate formation of the N-(alkoxycarbonyl)phenylaminomethylphenyl impurities during condensation. (3) Undesirable side reactions such as sulfonation of the N-aryl carbamates and chlorination which occur with sulfuric and hydrochloric acid for example, are avoided. Since these side reactions are suppressed, a higher quality diphenylmethane dicarbamate and polymethylene polyphenyl carbamate product is obtained which upon pyrolysis to the polymeric aromatic isocyanate gives a higher isocyanate yield.